Palladium metal and alloys containing palladium are used extensively as protective coatings on a variety of articles and devices both for aesthetic reasons and for utilitarian reasons. Often, decorative articles such as jewelry, watches, etc. are coated with palladium metal or palladium alloy coatings to produce a bright, shiny surface attractive to the user. Also, such coated surfaces remain bright and shiny over long periods of time because of the chemical inertness of palladium coatings.
An extremely important use for palladium metal and alloys is for electrical contact surfaces in electrical contact devices and electrical connectors. Palladium is ideally suited for such applications because of its high electrical conductivity and because of its chemical inertness. Early electrical contact devices and electrical connectors used palladium metal and palladium alloys in the form of wrought metal or alloys or clad inlays often as a replacement for gold electrical contacts. More recently, considerable emphasis has been put on fabrication of electrical contact devices by electrodeposition of palladium metal and palladium alloys since electrodeposition is generally more convenient and a less expensive process for producing electrical contact devices and electrical connectors.
The development of a satisfactory electrodeposition process for palladium metal and palladium alloy has not proved to be easy. Despite extensive experimentation in this technology, electrodeposited palladium generally was not adherent, tended to be porous, often developed cracks and generally was quite brittle. Such a product generally was not satisfactory for use as electrical contacts in electrical devices and often left much to be desired for decorative articles.
It was quickly discovered that much of the problem in palladium electroplating was due to the incorporation of hydrogen in the palladium. Hydrogen is often a byproduct of palladium electroplating because of the close proximity of the potential for electrolyzing water to the plating potential for electroplating palladium. Incorporation of hydrogen into the palladium degrades many of the desirable properties of palladium metal such as ductility, adhesion, etc. Indeed, many palladium electroplating processes appeared to work well in the laboratory where the plating potential could be precisely controlled and plating rates are relatively low. However, under commercial manufacturing conditions, these processes proved unreliable either because the plating potential was not precisely controlled or because increasing the plating rate to that required in a commercial electroplating process necessitated plating potentials that led to the evolution of hydrogen during the electroplating process.
A major advance in palladium electroplating technology occurred with the discovery that certain palladium complex ions exhibited electroplating potentials far removed from the hydrogen evolution potential. The complexing agents involve certain aliphatic polyamines with best results obtained with 1,3 diamino propane. This work is described in U.S. Pat. No. 4,486,274 issued to J. A. Abys, et al on Dec. 4, 1984.
This discovery led to a major commercial effect in palladium electroplating. The process has been used extensively in the United States and throughout the world to electroplate palladium typically for electrical contact surfaces in various devices such as electrical connectors. It has generally been used in applications formerly requiring gold contact surfaces and has led to considerable cost savings because of the lower cost of palladium as compared to gold. Further development work has been done as described in such references as U.S. Pat. No. 4,468,296 issued to J. A. Abys et al on Aug. 28, 1984 (replenishment compound for a palladium electroplating process) and U.S. Pat. No. 4,493,754 issued to J. A. Abys et al on Jan. 15, 1985 (unique anode structure for use in palladium electroplating process). Often, the palladium layer of the contact surface is covered with a very this layer of gold to improve wear characteristics.
Because of the success of the palladium electroplating process involving aliphatic amines, further improvements both in the electroplating process and properties of the electroplated palladium have become desirable. In particular, cost reduction in the palladium electroplating process is desirable as is greater versatility in the choice of palladium electroplating species. Also, greater ductility and adhesion of the electroplated palladium is desirable particularly for relatively thick (greater than 2.5-5.0 .mu.m) layers. Such thick layers of palladium metal and palladium alloys would be highly useful for devices where extended wear is required.
Great adhesion and ductility is also required in stripe on strip connector manufacturing operations. Here, the contact metal (e.g. palladium) is electroplated as a stripe on a wide strip of substrate (e.g. a copper alloy) and this substrate with the stripe of contact metal punched and formed into connector pins. The electroplated contact metal stripe is placed on the strip in such a position and location that after the contact pin is formed the contact metal is located at the exact point where electrical contact is made with the mating contact structure.
Stripe on strip manufacturing operations for electrical contacts have a number of advantages. First of all, high speed plating procedures can be used to produce the plated strip rapidly and cheaply. Reel-to-reel continuous strip plating processes can be used which often produces high throughput at relatively low cost. Also, the same plated strip can be used for many different connectors and connector pins.
Because of the stamping operation on the plated strip, the electroplated palladium or palladium alloy must be highly adherent to the substrate material and extremely ductile, and remain crack-free and porosity-free after the stamping operation.
A variety of references have disclosed palladium electroplating processes including U.S. Pat. No. 4,487,665 issued to K. B. Miscioscio et al on Dec. 11, 1984; U.S. Pat. No. 4,491,507 issued to G. Herklotz et al on Jan. 1, 1985 and U.S. Pat. No. 4,545,869 issued to I. Goldman on Oct. 5, 1985. The palladium tetra-ammine complex is used as the source of palladium in a number of palladium electroplating processes including those described in U.S. Pat. No. 4,622,110 issued to J. L. Martin et al on Nov. 11, 1986; U.S. Pat. No. 4,552,628 issued to J. Wilcox on Nov. 12, 1985 and U.S. Pat. No. 4,628,165 issued to F. I. Nobel on Dec. 9, 1986.